A full text digital library of nearly 80,000 books on theology and religion, all published before 1923 (i.e. no longer protected by copyright). This is digital project maintained at Princeton theological seminary.
Searchable Latin texts of the complete Augustine corpus. One student copied a section of Latin from this site into Google translator (the work is not available in a published English translation) and the translation was decent.
The CSEL is intended to include the ecclesiastical authors who wrote in Latin from the late 2nd century AD until the death of Bede in 735. The texts are edited on the basis of all extant manuscripts and according to the principles of modern textual criticism and thus aim to provide a critical replacement for the corresponding volumes of the Patrologia Latina.
Somewhat eclectic collection of primary and secondary texts most of which are in English, but many in Greek and Latin. Figures of background interest like Josephus and Pliny are here, but no patristic authors. Contains a searchable copies of the Latin Vulgate and the Quran in English and Arabic. Serves up also the Duke Databank of Documentary Papyri.
Greek texts from Christian writers of the first 300 years. On this site (Hathitrust), some of the volumes of GCS are available full text and some are search only, and, in fact, theses are not searchable scans.
Index of Biblical Quotations and Allusions in Early Christian Literature, being the contents of the printed Biblical Patristica. "This site already allows simple interrogation in a corpus of about 400,000 biblical references."